1. Field of the Invention
The present invention relates generally to a vacuum-sealed capacitive pressure sensor and, more particularly, to a batch processed, vacuum-sealed capacitive pressure sensor where an internal electrode within the vacuum sealed chamber of the sensor makes electrical contact with an external electrode outside of the sealed chamber through a polysilicon, or other high temperature compatible conductive material, layer that forms the seal of the chamber.
2. Background of the Related Art
The need for highly sensitive, miniature-sized pressure transducers is important for incorporation into micro-electromechanical systems (MEMS) devices. Pressure transducers of this type have many applications, including uses in motor vehicles. One motor vehicle application requires the use of pressure sensors for measuring both ambient and subatmospheric pressure levels. In internal combustion engine applications, fine control of fuel metering has required that the rapidly-fluctuating pressure levels within the intake manifold of the engine be measured as well as the less-rapidly fluctuating ambient pressure levels. Sensors able to measure these pressures reliably and with adequate response time have been difficult to obtain and are typically expensive.
A majority of the currently employed pressure sensors are piezoresistive devices, well known to those skilled in the art. However, capacitive pressure sensor devices are becoming increasingly more of the focus in the industry because of their higher pressure sensitivity, lower temperature sensitivity, and reduced power consumption. Because piezoresistive devices can be more cheaply produced and are currently more reliable, they remain, however, the more popular of the known pressure sensors. Research continues on the capacitive pressure sensor devices to reduce their cost of manufacture and increase their reliability because of their inherent advantages.
Many variations of capacitive pressure sensors are known in the art. Capacitive pressure sensors typically measure pressure by the capacitive changes resulting from variations in the distance between a movable diaphragm and a substrate that occur because of pressure changes. A vacuum sealed chamber is defined in the sensor, where an internal electrode is formed on the substrate within the chamber and an external electrode is formed as part of the movable diaphragm. As the pressure outside of the chamber increases or decreases, the diaphragm moves towards or away from the substrate, and the charge on the electrodes changes giving an indication of the pressure change.
One area that has prevented capacitive pressure sensors from becoming more reliable is the ability to provide an electrical connection to the internal electrode within the sealed chamber in an inexpensive and reliable manner using batch processing techniques without effecting the seal integrity. Other types of semiconductor devices, such as accelerometers, gyroscopes, resonators, etc., incorporating a circuit or multiple electrodes within a sealed chamber could also benefit from such an electrical connection to the interior of a vacuum chamber. There has been much work in the industry to connect a lead to the internal electrode in an effective manner to overcome this problem. In one example, a hole is formed through a glass substrate to expose the electrode, and then a lead is electrically connected to the electrode through the hole, and the hole is then filled with an appropriate conductive filler material. U.S. Pat. Nos. 4,345,299 and 4,773,972 disclose two types of capacitive pressure sensors that make contact to the electrode within the sealed chamber in this manner.
While the various known techniques of connecting an external lead to the electrode within the chamber are generally successful in accomplishing this goal, these techniques allow considerable room for improvement. What is needed is a capacitive pressure sensor that is cost-effective, reliable and robust, and includes an improved technique for electrically connecting a lead to an electrode within a vacuum chamber associated with the transducer. It is therefore an objective of the present invention to provide such a sensor.